JP4617572B2

JP4617572B2 - Nitrogen-containing wastewater treatment method

Info

Publication number: JP4617572B2
Application number: JP2000394079A
Authority: JP
Inventors: 稔徳原; 総介西村; 博之藤井
Original assignee: Kurita Water Industries Ltd; Nippon Steel Nisshin Co Ltd
Current assignee: Kurita Water Industries Ltd; Nippon Steel Nisshin Co Ltd
Priority date: 2000-12-26
Filing date: 2000-12-26
Publication date: 2011-01-26
Anticipated expiration: 2020-12-26
Also published as: JP2002192189A

Description

【０００１】
【発明の属する技術分野】
本発明は窒素含有排水から窒素を生物学的脱窒処理により除去するようにした窒素含有排水の処理方法に関する。
【０００２】
【従来の技術】
アンモニア性または有機性窒素化合物を含む排水を処理する方法として、生物学的硝化脱窒処理法がある。この方法は活性汚泥により排水中のＣＯＤ，ＢＯＤ成分を分解するとともに、有機性窒素化合物をアンモニア性窒素とし、アンモニア性窒素を硝化細菌により硝酸性または亜硝酸性窒素に硝化（酸化）した後、脱窒細菌により窒素ガスに還元して脱窒する方法である。この処理法では、脱窒槽を前段に設け、硝化液と分離汚泥を脱窒槽に返送して原水と混合し、脱窒を行うと同時にＢＯＤ成分を分解する方法も行われている。
【０００３】
このような生物学的硝化脱窒法では、硝化工程において生成する硝酸性または亜硝酸性窒素を脱窒工程において脱窒細菌の作用により還元して窒素ガスに転換する際、脱窒細菌の硝酸呼吸に必要な水素の供与体を添加する必要がある。
この水素供与体としてメタノールのような基質を添加すると、処理コストが高くなるとともに、余剰汚泥が増加する。脱窒工程に原水または他の排水を導入して、そのＢＯＤ成分を利用する場合でも、そのままの状態で排出すると処理水にアンモニウムイオンがそのまま残留するため、最終脱窒工程には窒素を含まない有機物を系外から添加する必要がある。
【０００４】
このような点を解決する方法として、特公昭５９−４８６７７号には、例えば窒素除去系から排出される余剰汚泥を熱アルカリで分解し、その可溶化液を脱窒工程に導入して水素供与体として用いる方法が提案されている。
また特開平８−１１９０号には、脱窒処理にオゾン処理を組み込むことにより、余剰汚泥の発生量を削減できるとともに、メタノールなどのＢＯＤ源の使用量を低減できる排水の生物学的窒素除去方法が記載されている。
【０００５】
上記従来の方法では、アルカリ処理やオゾン処理した改質汚泥を脱窒反応の水素供与体として利用しているので、脱窒反応に添加するＢＯＤ源の使用量を低減することができるが、脱窒反応においてはＮＯxが電子受容体として働くので、脱窒槽内の残留ＮＯx濃度が著しく低い状態では改質汚泥の分解反応が阻害され、十分な汚泥減量効果は得られないという問題点がある。
【０００６】
一方、脱窒槽内にＮＯxを存在させることにより上記問題点の解決を図れば、ＮＯxが沈殿槽に流入し、沈殿槽内で脱窒反応により窒素ガスが発生し、窒素ガスが汚泥に付着して汚泥浮上の問題を引き起こす。この汚泥浮上が発生すると、処理水ＳＳ濃度が上昇悪化し、沈殿槽水面にスカム・発泡を引き起こし、甚だしい場合には大量の汚泥が処理水に流出して脱窒処理に必要な汚泥量を系内に維持できなくなる場合もある。
【０００７】
【発明が解決しようとする課題】
本発明の課題は、上記二律背反を解決するため、脱窒のための有機物の添加量を削減することができるとともに、余剰汚泥の生成量を効率よく減少させることができ、しかも安定して高水質の処理水を得ることができる窒素含有排水の処理方法を提案することである。
【０００８】
【課題を解決するための手段】
本発明は次の窒素含有排水の処理方法である。
（１）ＮＯx−Ｎを含有する窒素含有排水を、脱窒槽を含む窒素除去系において処理する方法において、
ＮＯx−Ｎを含有する排水を２個以上の脱窒槽に導入して窒素を生物学的に脱窒する脱窒工程と、
窒素除去系において生成する生物汚泥の一部を引き抜いて易生物分解性に改質処理したのち、この改質汚泥を前記脱窒槽に導入する改質工程とを含み、
前記脱窒工程の前段の脱窒槽に排水および改質汚泥を導入し、ＢＯＤ流入量を脱窒槽に流入するＮＯx−Ｎ負荷の２．５倍以下となるように制御することにより、前段の脱窒槽流出水のＮＯx−Ｎ濃度が５〜１００ｍｇ−Ｎ／Ｌとなるように脱窒し、
前段の脱窒槽の脱窒液を後段の脱窒槽に導入するとともに、ＢＯＤ源として改質汚泥以外の基質を注入して、後段の脱窒槽流出水のＮＯx−Ｎ濃度が１０ｍｇ−Ｎ／Ｌ以下となるように、残留ＮＯx−Ｎを脱窒する
窒素含有排水の処理方法。
（２）脱窒槽へのＢＯＤ流入量を改質汚泥の流入量および／または基質の注入量により調節する上記（１）記載の処理方法。
（３）改質処理がオゾン処理である上記（１）または（２）記載の処理方法。
（４）直列に設けられた少なくとも２個の脱窒槽を使用して脱窒工程を行い、前段の脱窒槽において脱窒槽流出水のＮＯx−Ｎ濃度が５〜１００ｍｇ−Ｎ／Ｌとなるように脱窒し、後段の脱窒槽において脱窒槽流出水のＮＯx−Ｎ濃度が１０ｍｇ−Ｎ／Ｌ以下となるように脱窒する上記（１）ないし（３）のいずれかに記載の処理方法。
【０００９】
本明細書において「ＮＯx−Ｎ」は「硝酸性窒素」および／または「亜硝酸性窒素」を意味する。また「ＮＯx」は「硝酸イオン」および／または「亜硝酸イオン」を意味する。さらに「基質」とは、水素供与体として脱窒反応に添加するＢＯＤ源のうちの改質汚泥以外のもの、例えばメタノールや酢酸、あるいはＢＯＤ成分を含む原水や他の排水などである。
【００１０】
本発明の処理対象とする排水は、硝酸および／または亜硝酸を含有する排水であり、このほか有機物、その他の不純物を含んでいてもよい。排水中に有機性窒素化合物またはアンモニア性窒素化合物が含まれている場合は、有機物分解工程を設けて好気性または嫌気性下に処理し、活性汚泥により排水中のＣＯＤ、ＢＯＤ成分を分解するとともに、有機性窒素化合物をアンモニア性窒素とし、硝化工程を設けてアンモニア性窒素を硝化細菌により好気性下に硝酸性または亜硝酸性窒素に硝化（酸化）して、脱窒工程に供することができる。有機物分解工程および硝化工程は従来のものと同様の方法を採用することができる。このような窒素除去系において、各工程の組合せあるいは回数等は排水に応じて自由に設定できる。例えば、脱窒槽の前段に硝化槽を設け、硝化および脱窒を行う方法；脱窒槽を後段に硝化槽を設け、硝化液と分離汚泥を脱窒槽に返送して原水と混合し、脱窒と同時にＢＯＤの除去を行い、その後硝化、脱窒を行う方法；有機物分解工程として好気処理と嫌気処理を組み合せる方法等を採用することも可能である。
【００１１】
本発明における脱窒工程は、２個以上の脱窒槽を用いて脱窒を行うが、好ましくは２〜４個の脱窒槽を用いて行うのが望ましい。２個以上の脱窒槽を用いる場合、少なくとも２個の脱窒槽は直列に配置するのが好ましい。
【００１２】
まず、２個の脱窒槽を直列に配置して脱窒する場合について説明する。前段の脱窒槽では、排水および改質汚泥を導入し、この脱窒槽からの流出水のＮＯx濃度が５〜１００ｍｇ−Ｎ／Ｌ、好ましくは１０〜１００ｍｇ−Ｎ／Ｌとなるように生物学的に脱窒を行う。すなわち、前段の脱窒槽ではＮＯxが残留するように脱窒を行い、完全には脱窒を行わない。後段の脱窒槽では、前段の脱窒液を導入し、この後段の脱窒槽からの流出水のＮＯx濃度が１０ｍｇ−Ｎ／Ｌ以下、好ましくは５ｍｇ−Ｎ／Ｌ以下となるように生物学的に脱窒を行う。すなわち、後段の脱窒槽ではできるだけＮＯxが残留しないように脱窒を行う。
【００１３】
上記のような条件で脱窒を行うことにより、前段の脱窒槽では改質汚泥をＢＯＤ源として脱窒が進行するとともに、改質汚泥も効率よく無機化され、汚泥減量が良好に行われる。このため前段の脱窒槽には改質汚泥以外のＢＯＤ源の添加は通常不要であるが、添加することもできる。
【００１４】
後段の脱窒槽ではできるだけＮＯxが残留しないように脱窒を行うことにより、後工程の沈殿槽で脱窒現象による汚泥浮上を防止する。脱窒現象による汚泥浮上を防止するためには、ＮＯx濃度を、沈殿槽内に溶存酸素が存在しない場合はほぼゼロ、溶存酸素が存在する場合（例えば脱窒槽と沈殿槽の間に再曝気槽を備えている場合）でも１０ｍｇ−Ｎ／Ｌ以下とする。
【００１５】
次に、前段の脱窒槽内のＮＯx濃度を５〜１００ｍｇ−Ｎ／Ｌに制御する方法について説明する。脱窒槽内の残留ＮＯx濃度の決定に関与する要素はＮＯx含有排水の流入と、脱窒反応に必要なＢＯＤ源の流入である。この両者のバランスを維持するため、流入するＮＯxに比例した量のＢＯＤを注入し、その比率がＢＯＤ／Ｎ＝２．５〜４程度であれば、ＮＯxの除去が良好に行われることが知られている。本発明ではこのような従来の知見を参考に、前段の脱窒槽の槽内液のＢＯＤ／Ｎ比を２．５以下となるように制御すれば、脱窒槽内の残留ＮＯx濃度がゼロとならず、５〜１００ｍｇ−Ｎ／Ｌに維持することができる。
【００１６】
前段の脱窒槽内のＢＯＤ／Ｎ比を２．５以下に制御するには、主として改質汚泥の導入量を調節することにより制御するが、場合によってはＢＯＤ源としてメタノールなどの分解速度の速い基質も使用することとし、この注入量を制御したり、ＮＯx−Ｎを注入する方法などを併用することもできる。
ＢＯＤ／Ｎ比が２．５以下のいずれの値を採用するかについては、事前の実験や試運転を行って決定することが好ましい。このような実験、試運転においては、ＢＯＤ／Ｎ比を変更しながら運転を行い、実際に脱窒槽内のＮＯx濃度が５〜１００ｍｇ−Ｎ／Ｌ、好ましくは１０〜１００ｍｇ−Ｎ／ＬとなるＢＯＤ／Ｎ比を見い出せばよい。
【００１７】
後段の脱窒槽ではＢＯＤ／Ｎ比を２．５〜４に制御することにより、十分にＮＯxを除去して沈殿槽での汚泥浮上を防止する。
後段の脱窒槽には、ＢＯＤ源としてメタノールなどの分解速度の速い改質汚泥以外の基質を注入して、脱窒槽内のＮＯx濃度を積極的に制御するのが好ましい。また硝酸排水を後段の脱窒槽に導入してもよい。
【００１８】
３個以上の脱窒槽を用いて脱窒工程を行う場合は、沈殿槽に最も近い脱窒槽において、この脱窒槽からの流出水のＮＯx濃度が１０ｍｇ−Ｎ／Ｌ以下、好ましくは５ｍｇ−Ｎ／Ｌ以下となるように生物学的に脱窒を行い、他の少なくとも１個の脱窒槽では脱窒槽からの流出水のＮＯx濃度が５〜１００ｍｇ−Ｎ／Ｌ、好ましくは１０〜１００ｍｇ−Ｎ／Ｌとなるように生物学的に脱窒を行うのが望ましい。脱窒槽からの流出水のＮＯx濃度の制御は前記と同じ方法により制御することができる。
【００２０】
脱窒工程により窒素を除去した液は、残留する有機物を除去するために、必要に応じて再曝気を行った後、固液分離を行って、生物汚泥を分離し、分離液を処理水として排出する。
【００２１】
本発明では、上記のような脱窒工程を含む窒素除去系において生成する生物汚泥の少なくとも一部を引き抜き、改質処理することにより汚泥を易生物分解性に分解してＢＯＤ化した後、脱窒槽に導入する。
改質処理する生物汚泥は、脱窒工程のほか、固液分離工程、あるいは排水の種類に応じて必要により設けられる有機物分解工程または硝化工程などから引き抜くこともできる。
【００２２】
具体的な引抜位置としては、脱窒槽のほか、脱窒槽の前段または後段に設けられた硝化槽、曝気槽もしくは沈殿槽などがあげられる。脱窒槽を複数設けた場合、どの脱窒槽から引き抜いてもよいが、後段側の脱窒槽から引き抜くのが好ましい。改質処理するために引き抜く生物汚泥は濃縮された状態であってもよく、また希薄な状態であってもよい。引抜汚泥はそのまま改質処理してもよく、遠心分離機などによりさらに高濃度に濃縮してもよい。
【００２３】
生物汚泥を引き抜く量は、基本的には１日あたり、系内に保有される汚泥固形物（ＶＳＳ）量の１／２０〜１／３の量とし、引き抜く対象の工程の能力、汚泥の性状、および改質処理の程度等に応じて調整できる。
【００２４】
引抜汚泥を生物が分解し易い性状に改質する改質処理方法としては、任意の方法を採用することができる。例えば、オゾン処理による改質処理、酸処理による改質処理、アルカリ処理による改質処理、加熱処理による改質処理、高圧パルス放電処理、ボールミル、コロイドミル等のミルによる磨砕処理、これらを組み合せた改質処理等を採用することができる。これらの中ではオゾン処理による改質処理が、処理操作が簡単かつ処理効率が高いため好ましい。
【００２５】
改質処理としてのオゾン処理は、引き抜いた生物汚泥とオゾンを例えば常温で接触させることにより行うことができる。接触方法としては、オゾン処理槽に汚泥を導入してオゾンを吹込む方法などが採用できる。オゾンとしてはオゾンガスの他、オゾン含有空気、オゾン化空気などが使用できる。
このオゾン処理においては、オゾンを生物汚泥に対して２〜１０重量％の範囲内で反応させるのが好ましい。こうしてオゾン処理された汚泥は、対汚泥あたりのオゾン注入率が０．０２ｍｇ−オゾン／ｍｇ−ＳＳ以上では、生物汚泥の細胞壁の糖鎖長が小さくなって生分解性が非常に向上する。
【００２６】
オゾン処理に際しては生物汚泥をｐＨ５以下に調整すると、オゾン使用量を減少させることができる。その場合、オゾン処理後に再度ｐＨ調整して脱窒工程に導入することにより、脱窒細菌の活性を維持することが可能である。
【００２７】
改質処理としての酸処理では、引抜汚泥を改質槽に導き、塩酸、硫酸などの鉱酸を加え、ｐＨ２．５以下、好ましくはｐＨ１〜２の酸性条件下で所定時間滞留させればよい。滞留時間としては、例えば５〜２４時間とする。この際、汚泥を加熱、例えば５０〜１００℃に加熱すると改質が促進されるので好ましい。このような酸による処理により汚泥は易生物分解性に改質される。
【００２８】
また、汚泥の改質処理としてのアルカリ処理では、引抜汚泥を改質槽に導き、水酸化ナトリウム、水酸化カリウム等のアルカリを汚泥に対して０．１〜１重量％加え、所定時間滞留させればよい。滞留時間は０．５〜２時間程度で汚泥は易生物分解性に改質される。この際、汚泥を加熱し、例えば５〜１００℃に加熱すると改質が促進されるので好ましい。
【００２９】
改質処理としての加熱処理は、加熱処理単独で行うこともできるが、酸処理またはアルカリ処理と組み合せて行うのが好ましい。加熱処理単独で行う場合は、例えば温度７０〜１００℃、滞留時間２〜３時間とすることができる。
【００３０】
高電圧のパルス放電処理は、電極間隔３〜１０ｍｍ、好ましくは４〜８ｍｍのタングステン／トリウム合金等の＋極と、ステンレス鋼等の−極間に汚泥を存在させ、印加電気１０〜５０ｋＶ、好ましくは２０〜４０ｋＶ、パルス間隔２０〜８０Ｈｚ、好ましくは４０〜６０Ｈｚでパルス放電を行い、汚泥は順次循環させながら処理を行うことができる。
【００３１】
このようにして改質処理された改質処理汚泥は前段の脱窒槽に導入する。この場合、導入量を調節するなど、前記方法により前段の脱窒槽流出水のＮＯx−Ｎ濃度が５〜１００ｍｇ−Ｎ／Ｌとなるように制御する。脱窒槽が複数あるので、上記以外の脱窒槽に導入することもできる。
【００３２】
本発明の方法では、改質処理した汚泥を前段の脱窒槽に導入するとともに、前段の脱窒槽流出水のＮＯx−Ｎ濃度が５〜１００ｍｇ−Ｎ／Ｌとなるように制御するので、前段の脱窒槽における改質汚泥の分解反応が阻害されることはない。このため改質汚泥の分解速度は速く、汚泥減量が良好に行われ、排出される余剰汚泥量は減少し、場合によっては余剰汚泥量をゼロにすることも可能である。
また改質汚泥を水素供与体として効率よく脱窒が進行するので、メタノール等の基質を系外から添加する必要がなくなるか、あるいは添加量が減少する。
さらに、脱窒槽が複数設けられているため、後段の脱窒槽において残留する硝酸イオンおよび亜硝酸イオンをできるだけ除去することにより、沈殿槽における汚泥浮上が防止され、安定して高水質の処理水を得ることができる。
【００３３】
【発明の効果】
本発明の窒素含有排水の処理方法は、脱窒槽流出水のＮＯx濃度が特定の濃度となるように脱窒している前段の脱窒槽に改質汚泥を導入して脱窒し、前段の脱窒槽の脱窒液を後段の脱窒槽に導入するとともに、ＢＯＤ源として改質汚泥以外の基質を注入して、後段の脱窒槽流出水のＮＯx濃度が特定濃度以下となるように脱窒するので、改質汚泥の前段の脱窒槽での分解反応を促進して脱窒のための有機物の添加量を削減することができるとともに、余剰汚泥の生成量を効率よく減少させることができ、しかも安定して高水質の処理水を得ることができる。
【００３４】
【発明の実施の形態】
以下、本発明を図面の実施例により説明する。
図１は実施例の窒素含有排水の処理方法を示す系統図であり、改質処理としてオゾン処理を採用し、直列に配置した２個の脱窒槽により脱窒する場合の例を示している。図１において、１は第１脱窒槽、２は第２脱窒槽、３は曝気槽、４は沈殿槽、５はｐＨ調整槽、６はオゾン処理槽である。
【００３５】
第１脱窒槽１に原水路１０から原水を導入し、沈殿槽４から返送汚泥路１１を通して返送される返送汚泥と混合するとともに、オゾン処理槽６からオゾン処理汚泥をオゾン処理汚泥路１２を通して導入し、嫌気性下で攪拌器１３により緩やかに攪拌して脱窒細菌により脱窒処理を行う。第１脱窒槽１では、導入するオゾン処理汚泥の量を調節することにより、第１脱窒槽１に流入するＢＯＤ量を第１脱窒槽１に流入するＮＯx−Ｎ負荷の２．５倍以下となるように制御し、これにより第１脱窒槽１から第２脱窒槽２に送られる液のＮＯx−Ｎ濃度が５〜１００ｍｇ−Ｎ／Ｌ、好ましくは１０〜１００ｍｇ−Ｎ／Ｌとなるように制御する。このようにして脱窒することにより、オゾン処理汚泥が効率よく分解され、しかも脱窒細菌がオゾン処理汚泥中のＢＯＤ成分を水素供与体として利用して増殖し、硝酸イオンおよび亜硝酸イオンを窒素ガスに還元して除去する。窒素ガスは排ガス路１４から排出する。
【００３６】
第１脱窒槽１の槽内液は連絡路１５から第２脱窒槽２に導入し、ＢＯＤ注入路１６からメタノールを注入し、嫌気性下で攪拌器１７により緩やかに攪拌して脱窒処理を行う。第２脱窒槽２では注入するメタノール量を調節することにより、第２脱窒槽２に流入するＢＯＤ量を第２脱窒槽２に流入するＮＯx−Ｎ負荷の２．５〜４倍となるように制御し、これにより第２脱窒槽２から曝気槽３に送られる液のＮＯx−Ｎ濃度が１０ｍｇ−Ｎ／Ｌ以下、好ましくは５ｍｇ−Ｎ／Ｌ以下となるように制御する。このようにして脱窒することにより、脱窒細菌がメタノールを水素供与体として利用して増殖し、残留する硝酸イオンおよび亜硝酸イオンをほぼ完全に窒素ガスに還元して除去するので、沈殿槽４における汚泥浮上が防止される。窒素ガスは排ガス路１８から排出する。
【００３７】
第２脱窒槽２の脱窒液は連絡路２０から曝気槽３に導入して好気処理し、残留有機物を分解する。
曝気液は連絡路２１から沈殿槽４に導入して固液分離し、分離汚泥の一部を返送汚泥路１１から第１脱窒槽１に返送し、分離液を処理水として処理水路２２から排出する。第２脱窒槽２においてほぼ完全に脱窒されているので、沈殿槽４における汚泥浮上は防止され、固液分離は良好に行われる。このため処理水質の悪化は生じない。
【００３８】
ｐＨ調整槽５では、沈殿槽４で固液分離された分離汚泥の他の一部を連絡路２３から導入し、ｐＨ５以下に調整する。
オゾン処理槽６では、ｐＨ調整された汚泥を連絡路２４から導入し、これをオゾン処理して可溶化しＢＯＤ化する。このオゾン処理汚泥をオゾン処理汚泥路１２から第１脱窒槽１に導入して脱窒を行うことにより、オゾン処理汚泥を脱窒細菌の硝酸呼吸に必要な水素供与体として利用する。なお余剰汚泥が生じる場合は、汚泥排出路２５から系外に排出する。
【００３９】
このように生物汚泥をオゾン処理したオゾン処理汚泥は、生物汚泥がＢＯＤ化して生分解性がよいため、オゾン処理汚泥路１２から第１脱窒槽１に導入し、かつ導入量を脱窒槽流出水のＮＯx−Ｎ濃度が前記濃度となるように制御すると、オゾン処理汚泥が第１脱窒槽１で効率よく分解され、しかも脱窒細菌の硝酸呼吸に必要な水素供与体として利用される。このためメタノール等の基質を系外から添加することなく脱窒工程を行うことができる。これにより処理コストが低下するとともに、余剰汚泥量が減少する。また第２脱窒槽２から曝気槽３に送られる液のＮＯx−Ｎ濃度が前記濃度となるように制御してさらに脱窒することにより、残留する硝酸イオンおよび亜硝酸イオンがほぼ完全に除去され、これにより沈殿槽４における汚泥浮上が防止され、安定して高水質の処理水を得ることができる。
【００４０】
【実施例】
次に本発明を実施例により説明する。
【００４１】
試験例１
脱窒槽内の残留ＮＯx濃度と、汚泥減量性能との関係を求める試験を、密閉式のバッチ実験槽を用いて行った。すなわちオゾンを用いて易生物分解性に改質した汚泥を用いて、その分解速度を測定し、硝酸濃度と汚泥分解速度との関係を明らかにした。改質汚泥の分解速度は、注入した硝酸濃度の減少速度から求めた。
結果を図２に示す。
【００４２】
図２の結果からわかるように、改質汚泥の分解速度は硝酸濃度が５ｍｇ−Ｎ／Ｌ未満では非常に遅く、５〜１０ｍｇ−Ｎ／Ｌの範囲では硝酸濃度の上昇に伴って分解速度が上昇し、１０ｍｇ−Ｎ／Ｌ以上ではほぼ一定の分解速度となり、それ以上硝酸濃度を上昇させても改質汚泥分解速度の上昇は見られない。
この試験結果から、脱窒槽の残留ＮＯx濃度を５ｍｇ−Ｎ／Ｌ以上、好ましくは１０ｍｇ−Ｎ／Ｌ以上に維持することにより、改質汚泥の分解が効率よく進行し、汚泥減量が良好に行われると判断することができる。
【００４３】
実施例１、比較例１
図１の装置を用いて、表１に示すステンレス洗浄排水の脱窒処理を行った。ただし、オゾン処理する汚泥は沈殿槽４の代わりに第２脱窒槽２から引き抜いた。
第１脱窒槽１容量は２４０Ｌ、第２脱窒槽２容量は３３０Ｌ、曝気槽３は１７０Ｌとした。
【００４４】
【表１】
表１排水組成

【００４５】
表１に示した窒素含有排水を窒素負荷６７０ｇ−Ｎ／ｄ（９６Ｌ／ｈ）にて脱窒処理した。ＢＯＤ源として、第１脱窒槽１および第２脱窒槽２全体としてＢＯＤ／Ｎ＝２．９（１９４０ｇ−ＢＯＤ／ｄ）に相当するメタノールを注入した。第１脱窒槽１、第２脱窒槽２および曝気槽３のｐＨは塩酸を用いて７．０に制御した。第２脱窒槽２の汚泥８００ｇ−ＶＳＳ／ｄをポンプで連続して引き抜き、ｐＨ調整槽５にて塩酸を用いてｐＨ３に調整した後オゾン処理槽６に導入し、１．６〜２．５％−オゾン／ＶＳＳのオゾンガスと反応させて汚泥改質処理を行い、この改質汚泥を第１脱窒槽１に戻した。沈殿槽４からの返送汚泥流量は１４０Ｌ／ｈとし、第１脱窒槽１に返送した。
【００４６】
メタノールの注入方法は、実施例１においては、第１脱窒槽１ではメタノール注入なしとし、第２脱窒槽２に１９４０ｇ−ＢＯＤ／ｄすべてのメタノールを注入した。この時、実施例１の第１脱窒槽１では以下の計算の通りＢＯＤ／Ｎ＝１．１とした。
オゾン処理したオゾン処理汚泥のＢＯＤは、分析の結果０．９ｇ−ＢＯＤ／ｇ−ＶＳＳであったので、第１脱窒槽１に流入させたＢＯＤは、
０．９×８００＝７２０ｇ−ＢＯＤ／ｄ
であり、一方第１脱窒槽１への窒素負荷は６７０ｇ−Ｎ／ｄであるので、
ＢＯＤ／Ｎ＝７２０／６７０＝１．１
【００４７】
また第２脱窒槽２では以下の計算の通りＢＯＤ／Ｎ＝４．３であった。
第２脱窒槽２への流入窒素
＝第１脱窒槽１の残留硝酸濃度測定値×（原水量＋返送汚泥流量）
＝８０ｍｇ−Ｎ／Ｌ × （９６Ｌ／ｈ＋１４０Ｌ／ｈ）
＝１８．９ｇ−Ｎ／ｈ
＝４５０ｇ−Ｎ／ｄ
メタノール注入比率
＝１９４０ｇ−ＢＯＤ ÷ ４５０ｇ−Ｎ
＝４．３
【００４８】
一方比較例１として、実施例１の第１脱窒槽１および第２脱窒槽２の代わりに、容量５７０Ｌの脱窒槽一槽に置き換えた装置を用いて、実施例１と同様の処理を行った。メタノールは実施例１と同じ量（１９４０ｇ−ＢＯＤ／ｄ）を脱窒槽に注入した。このときのメタノール注入比率はＢＯＤ／Ｎ＝２．９であった。
【００４９】
実施例１、比較例１ともに、それぞれの運転条件を一定に保ち、３０日間連続運転を行って、窒素除去性能と汚泥減量性能を比較した。種汚泥は同じ排水を処理する実装置から採取し、初期濃度を４０００ｍｇ−ＶＳＳ／Ｌとして処理を開始した。余剰汚泥の引き抜きは行わなかった。処理結果を表２に示す。
【００５１】
【表２】

【００５２】
表２の結果から、実施例１および比較例１ともに窒素除去性能は十分であったが、実施例１では汚泥濃度が運転開始時の濃度が維持されたのに対して、比較例１では著しい汚泥濃度の上昇が起き、沈殿槽の汚泥界面の上昇が起きた。そのまま放置すると汚泥と処理水の沈殿分離ができなくなることが予想され、汚泥を引き抜いて余剰汚泥を処理する必要があった。
【図面の簡単な説明】
【図１】実施例の窒素含有排水の処理方法の系統図である。
【図２】試験例１の結果を示すグラフである。
【符号の説明】
１第１脱窒槽
２第２脱窒槽
３曝気槽
４沈殿槽
５ｐＨ調整槽
６オゾン処理槽
１０原水路
１１返送汚泥路
１２オゾン処理汚泥路
１３、１７攪拌器
１４、１８排ガス路
１５、２０、２１、２３、２４連絡路
１６ＢＯＤ注入路
２２処理水路
２５汚泥排出路[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for treating nitrogen-containing wastewater in which nitrogen is removed from the nitrogen-containing wastewater by biological denitrification.
[0002]
[Prior art]
As a method for treating wastewater containing ammoniacal or organic nitrogen compounds, there is a biological nitrification denitrification treatment method. This method decomposes COD and BOD components in the wastewater with activated sludge, converts the organic nitrogen compound to ammonia nitrogen, and nitrifies (oxidizes) ammonia nitrogen to nitrate or nitrite nitrogen by nitrifying bacteria. This is a method of denitrification by reducing to nitrogen gas with denitrifying bacteria. In this treatment method, there is also a method in which a denitrification tank is provided in the previous stage, the nitrification liquid and the separated sludge are returned to the denitrification tank and mixed with raw water, denitrification is performed, and at the same time, the BOD component is decomposed.
[0003]
In such a biological nitrification / denitrification method, nitrate or nitrite nitrogen produced in the nitrification process is reduced by the action of the denitrification bacteria in the denitrification process and converted into nitrogen gas. The necessary hydrogen donor must be added.
When a substrate such as methanol is added as the hydrogen donor, the treatment cost increases and surplus sludge increases. Even when raw water or other wastewater is introduced into the denitrification process and its BOD component is used, ammonium ions remain in the treated water if discharged as it is, so the final denitrification process does not contain nitrogen. It is necessary to add organic matter from outside the system.
[0004]
As a method for solving such a problem, Japanese Patent Publication No. 59-48677 discloses, for example, surplus sludge discharged from a nitrogen removal system is decomposed with hot alkali, and the solubilized liquid is introduced into a denitrification step to supply hydrogen. A method for use as a body has been proposed.
JP-A-8-1190 discloses a biological nitrogen removal method for wastewater that can reduce the amount of surplus sludge generated by incorporating ozone treatment into the denitrification treatment and can reduce the amount of BOD sources such as methanol. Is described.
[0005]
In the above conventional method, since the modified sludge treated with alkali or ozone is used as a hydrogen donor for the denitrification reaction, the amount of BOD source added to the denitrification reaction can be reduced. Since NOx acts as an electron acceptor in the nitrogen reaction, there is a problem that the decomposition reaction of the reformed sludge is hindered in a state where the residual NOx concentration in the denitrification tank is extremely low, and a sufficient sludge reduction effect cannot be obtained.
[0006]
On the other hand, if the above problem is solved by making NOx present in the denitrification tank, NOx flows into the precipitation tank, nitrogen gas is generated by the denitrification reaction in the precipitation tank, and nitrogen gas adheres to the sludge. Cause sludge levitation problems. When this sludge floats up, the treated water SS concentration rises and deteriorates, causing scum and foaming on the surface of the sedimentation tank. In severe cases, a large amount of sludge flows into the treated water and the amount of sludge required for denitrification treatment is reduced. In some cases, it cannot be maintained.
[0007]
[Problems to be solved by the invention]
The object of the present invention is to solve the above-mentioned trade-off, so that it is possible to reduce the amount of organic matter added for denitrification, to efficiently reduce the amount of excess sludge produced, and to achieve stable high water quality. This is to propose a method for treating nitrogen-containing wastewater from which treated water can be obtained.
[0008]
[Means for Solving the Problems]
The present invention is the following method for treating nitrogen-containing wastewater.
(1) In a method of treating nitrogen-containing wastewater containing NOx-N in a nitrogen removal system including a denitrification tank,
A denitrification step of biologically denitrifying nitrogen by introducing wastewater containing NOx-N into two or more denitrification tanks;
A part of the biological sludge generated in the nitrogen removal system is pulled out to be easily biodegradable, and then the modified sludge is introduced into the denitrification tank.
Drainage and reformed sludge are introduced into the previous denitrification tank of the denitrification process, and the BOD inflow is controlled to be 2.5 times or less of the NOx-N load flowing into the denitrification tank. Denitrification so that the NOx-N concentration of the nitrogen tank effluent is 5 to 100 mg-N / L,
Introduce the denitrification liquid of the first-stage denitrification tank into the second-stage denitrification tank, and inject a substrate other than the modified sludge as a BOD source, and the NOx-N concentration of the second-stage denitrification tank effluent is 10 mg-N / L or less A method for treating nitrogen-containing wastewater that denitrifies residual NOx-N so that
(2) The processing method according to (1), wherein the amount of BOD inflow into the denitrification tank is adjusted by the amount of reformed sludge and / or the amount of substrate injected.
(3) The processing method according to (1) or (2), wherein the reforming treatment is an ozone treatment.
(4) The denitrification process is performed using at least two denitrification tanks provided in series so that the NOx-N concentration of the denitrification tank effluent is 5 to 100 mg-N / L in the previous denitrification tank. The treatment method according to any one of the above (1) to (3), wherein denitrification is performed so that the NOx-N concentration in the denitrification tank effluent is 10 mg-N / L or less in a subsequent denitrification tank.
[0009]
In the present specification, “NOx—N” means “nitric nitrogen” and / or “nitrite nitrogen”. “NOx” means “nitrate ion” and / or “nitrite ion”. Further, the “substrate” is a BOD source to be added to the denitrification reaction as a hydrogen donor other than the modified sludge, such as methanol, acetic acid, or raw water or other waste water containing a BOD component.
[0010]
The wastewater to be treated in the present invention is wastewater containing nitric acid and / or nitrous acid, and may further contain organic substances and other impurities. When organic nitrogen compounds or ammonia nitrogen compounds are contained in the wastewater, an organic matter decomposition step is provided to treat it under aerobic or anaerobic conditions, and the activated sludge decomposes COD and BOD components in the wastewater. The organic nitrogen compound can be converted to ammonia nitrogen, and a nitrification process can be provided to nitrify (oxidize) the ammonia nitrogen to nitrate or nitrite nitrogen under aerobic conditions using a nitrifying bacterium, which can be used for the denitrification process. . The organic substance decomposition step and the nitrification step can employ the same methods as those used in the prior art. In such a nitrogen removal system, the combination or number of times of each process can be freely set according to the drainage. For example, a method in which a nitrification tank is provided in the front stage of the denitrification tank and nitrification and denitrification are performed; a nitrification tank is provided in the latter stage, the nitrification liquid and the separated sludge are returned to the denitrification tank and mixed with raw water, It is also possible to adopt a method in which BOD is removed at the same time, followed by nitrification and denitrification; a method in which aerobic treatment and anaerobic treatment are combined as an organic matter decomposition step.
[0011]
Denitrification step in the present invention is carried out denitrification by using two or more denitrification tank, preferably carried out using a 2-4 denitrification desirable. When using two or more denitrification tanks, it is preferable to arrange at least two denitrification tanks in series.
[0012]
First, the case where two denitrification tanks are arranged in series to perform denitrification will be described. In the preceding denitrification tank, waste water and modified sludge are introduced, and the NOx concentration of the effluent from this denitrification tank is 5-100 mg-N / L, preferably 10-100 mg-N / L. Perform denitrification. That is, denitrification is performed so that NOx remains in the previous denitrification tank, and denitrification is not performed completely. In the latter-stage denitrification tank, the first-stage denitrification liquid is introduced, and the NOx concentration of the effluent from this latter-stage denitrification tank is 10 mg-N / L or less, preferably 5 mg-N / L or less. Perform denitrification. That is, denitrification is performed so that NOx does not remain as much as possible in the subsequent denitrification tank.
[0013]
By performing denitrification under the conditions as described above, denitrification proceeds using the reformed sludge as a BOD source in the preceding denitrification tank, and the reformed sludge is also efficiently mineralized, and sludge reduction is performed well. For this reason, it is usually unnecessary to add a BOD source other than the modified sludge to the preceding denitrification tank, but it can also be added.
[0014]
By performing denitrification so that NOx does not remain as much as possible in the subsequent denitrification tank, sludge floating due to the denitrification phenomenon is prevented in the subsequent precipitation tank. In order to prevent sludge levitation due to the denitrification phenomenon, the NOx concentration is almost zero when there is no dissolved oxygen in the precipitation tank, and when there is dissolved oxygen (for example, a re-aeration tank between the denitrification tank and the precipitation tank) Even if it is provided), it should be 10 mg-N / L or less.
[0015]
Next, a method for controlling the NOx concentration in the preceding denitrification tank to 5 to 100 mg-N / L will be described. Factors involved in determining the residual NOx concentration in the denitrification tank are the inflow of NOx-containing wastewater and the inflow of BOD sources necessary for the denitrification reaction. In order to maintain a balance between the two, an amount of BOD proportional to the inflowing NOx is injected, and if the ratio is about BOD / N = 2.5-4, it is known that the removal of NOx is performed well. It has been. In the present invention, with reference to such conventional knowledge, if the BOD / N ratio of the liquid in the preceding denitrification tank is controlled to be 2.5 or less, the residual NOx concentration in the denitrification tank becomes zero. 5 to 100 mg-N / L.
[0016]
In order to control the BOD / N ratio in the preceding denitrification tank to 2.5 or less, it is mainly controlled by adjusting the amount of reformed sludge introduced. In some cases, the BOD source has a high decomposition rate of methanol or the like. A substrate is also used, and this injection amount can be controlled or a method of injecting NOx-N can be used in combination.
Which value of the BOD / N ratio is 2.5 or less is preferably determined by conducting a prior experiment or trial operation. In such experiments and test operations, operation is performed while changing the BOD / N ratio, and the NOx concentration in the denitrification tank is actually 5 to 100 mg-N / L, preferably 10 to 100 mg-N / L. Find the BOD / N ratio.
[0017]
In the subsequent denitrification tank, the BOD / N ratio is controlled to 2.5 to 4 to sufficiently remove NOx and prevent sludge floating in the precipitation tank.
It is preferable to positively control the NOx concentration in the denitrification tank by injecting a substrate other than the modified sludge having a high decomposition rate such as methanol as a BOD source into the subsequent denitrification tank. Moreover, you may introduce | transduce nitric acid waste_water | drain to a denitrification tank of a back | latter stage.
[0018]
When performing the denitrification process using three or more denitrification tanks, the NOx concentration of the effluent water from this denitrification tank is 10 mg-N / L or less, preferably 5 mg-N / L in the denitrification tank closest to the precipitation tank. Biological denitrification is carried out so that it is less than or equal to L, and in at least one other denitrification tank, the NOx concentration of the effluent from the denitrification tank is 5 to 100 mg-N / L, preferably 10 to 100 mg-N. It is desirable to biologically denitrify so as to be / L. The NOx concentration of the effluent water from the denitrification tank can be controlled by the same method as described above.
[0020]
The liquid from which nitrogen has been removed by the denitrification process is subjected to re-aeration as necessary in order to remove residual organic matter, followed by solid-liquid separation to separate biological sludge, and the separated liquid is used as treated water. Discharge.
[0021]
In the present invention, at least a part of the biological sludge generated in the nitrogen removal system including the denitrification step as described above is extracted and subjected to a reforming treatment to decompose the sludge to be readily biodegradable to form a BOD, and then remove it. Introduce into the nitrogen tank.
In addition to the denitrification process, the biological sludge to be reformed can be extracted from a solid-liquid separation process, an organic matter decomposition process or a nitrification process provided as necessary according to the type of waste water.
[0022]
Specific extraction positions include, in addition to the denitrification tank, a nitrification tank, an aeration tank, or a precipitation tank provided at the front stage or the rear stage of the denitrification tank. When a plurality of denitrification tanks are provided, they may be extracted from any denitrification tank, but are preferably extracted from the denitrification tank on the rear stage side. The biological sludge withdrawn for the modification treatment may be in a concentrated state or in a lean state. The extracted sludge may be reformed as it is, or may be concentrated to a higher concentration by a centrifugal separator or the like.
[0023]
The amount of biological sludge to be withdrawn is basically 1/20 to 1/3 of the amount of solid sludge (VSS) held in the system per day. , And the degree of reforming treatment can be adjusted.
[0024]
Any method can be adopted as a reforming treatment method for reforming the extracted sludge so as to be easily decomposed by organisms. For example, modification treatment by ozone treatment, modification treatment by acid treatment, modification treatment by alkali treatment, modification treatment by heat treatment, high pressure pulse discharge treatment, grinding treatment by a mill such as a ball mill, colloid mill, etc. Reforming treatment can be employed. Among these, the modification treatment by ozone treatment is preferable because the treatment operation is simple and the treatment efficiency is high.
[0025]
The ozone treatment as the reforming treatment can be performed by bringing the extracted biological sludge and ozone into contact with each other at room temperature, for example. As a contact method, a method of introducing sludge into an ozone treatment tank and blowing ozone in can be employed. As ozone, ozone gas, ozone-containing air, ozonized air, or the like can be used.
In this ozone treatment, it is preferable to react ozone within a range of 2 to 10% by weight with respect to biological sludge. The sludge thus treated with ozone has a biodegradability that is greatly improved because the sugar chain length of the cell wall of the biological sludge becomes small when the ozone injection rate per sludge is 0.02 mg-ozone / mg-SS or more.
[0026]
If the biological sludge is adjusted to pH 5 or lower during ozone treatment, the amount of ozone used can be reduced. In that case, it is possible to maintain the activity of denitrifying bacteria by adjusting the pH again after ozone treatment and introducing it into the denitrification step.
[0027]
In the acid treatment as the reforming treatment, the drawn sludge is introduced into the reforming tank, mineral acid such as hydrochloric acid or sulfuric acid is added, and the slurry is retained for a predetermined time under acidic conditions of pH 2.5 or less, preferably pH 1-2. . The residence time is, for example, 5 to 24 hours. At this time, it is preferable to heat the sludge, for example, to 50 to 100 ° C., because the reforming is promoted. By treatment with such an acid, sludge is easily biodegradable.
[0028]
Moreover, in the alkali treatment as sludge reforming treatment, the drawn sludge is guided to a reforming tank, and alkali such as sodium hydroxide or potassium hydroxide is added to the sludge in an amount of 0.1 to 1% by weight, and is retained for a predetermined time. Just do it. The residence time is about 0.5 to 2 hours, and the sludge is easily biodegradable. At this time, it is preferable to heat the sludge, for example, to 5 to 100 ° C., since the reforming is promoted.
[0029]
The heat treatment as the reforming treatment can be performed alone, but is preferably performed in combination with acid treatment or alkali treatment. In the case of performing the heat treatment alone, for example, the temperature can be set to 70 to 100 ° C. and the residence time can be set to 2 to 3 hours.
[0030]
In the high voltage pulse discharge treatment, sludge is present between a positive electrode such as tungsten / thorium alloy having an electrode interval of 3 to 10 mm, preferably 4 to 8 mm, and a negative electrode such as stainless steel, and an applied electricity of 10 to 50 kV, preferably Can perform pulse discharge at 20 to 40 kV and a pulse interval of 20 to 80 Hz, preferably 40 to 60 Hz, and the sludge can be processed while being circulated sequentially.
[0031]
In this way, the processed reforming the reforming treatment sludge is introduced in front of the denitrification tank. In this case, such as adjusting the introduction amount, NOx-N concentration in the denitrification tank effluent preceding it is controlled to be 5 ~100 mg-N / L by the method. Since there are a plurality of denitrification tanks, they can be introduced into other denitrification tanks.
[0032]
In the method of the present invention, together with introducing the sludge treated reformed in front of the denitrification tank, and controls so as NOx-N concentration of the preceding denitrification tank effluent is 5 ~100 mg-N / L, front The decomposition reaction of the modified sludge in the denitrification tank is not hindered. Therefore, the degradation rate of the modified sludge is fast, the sludge reduction is performed well, the amount of excess sludge discharged is reduced, and in some cases, the amount of excess sludge can be made zero.
Further, since denitrification proceeds efficiently using the modified sludge as a hydrogen donor, it is not necessary to add a substrate such as methanol from the outside of the system, or the amount of addition is reduced.
Furthermore, since the denitrification tank is provided with a plurality of, by and Turkey be removed as much as possible nitrate ion and nitrite ion remaining in the later stage of denitrification tank, the sludge floating is prevented in the precipitation tank, a stable processing of high quality You can get water.
[0033]
【The invention's effect】
Method of treating nitrogen-containing wastewater of the present invention is to introduce a modified sludge in front of the denitrification tank in which the NOx concentration of the denitrification tank effluent water is denitrified so that the specific concentration denitrification, preceding de Since the denitrification liquid from the nitrogen removal tank is introduced into the subsequent denitrification tank and a substrate other than the modified sludge is injected as a BOD source, the NOx concentration in the downstream denitrification tank effluent is denitrified so that it is below a specific concentration . In addition, it is possible to reduce the amount of organic matter added for denitrification by accelerating the decomposition reaction of the modified sludge in the previous denitrification tank, and to reduce the amount of excess sludge generated efficiently and stably. As a result, high-quality treated water can be obtained.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described with reference to the drawings.
FIG. 1 is a system diagram showing a method for treating nitrogen-containing wastewater according to an embodiment, and shows an example in which ozone treatment is adopted as reforming treatment and denitrification is performed by two denitrification tanks arranged in series. In FIG. 1, 1 is a first denitrification tank, 2 is a second denitrification tank, 3 is an aeration tank, 4 is a precipitation tank, 5 is a pH adjustment tank, and 6 is an ozone treatment tank.
[0035]
Raw water is introduced into the first denitrification tank 1 from the raw water channel 10, mixed with the return sludge returned from the sedimentation tank 4 through the return sludge channel 11, and ozone-treated sludge is introduced from the ozone treatment tank 6 through the ozone-treated sludge channel 12. Under anaerobic conditions, the mixture is gently stirred by the stirrer 13 and denitrified by denitrifying bacteria. In the first denitrification tank 1, the amount of BOD flowing into the first denitrification tank 1 is adjusted to 2.5 times or less of the NOx-N load flowing into the first denitrification tank 1 by adjusting the amount of ozone-treated sludge to be introduced. So that the NOx-N concentration of the liquid sent from the first denitrification tank 1 to the second denitrification tank 2 is 5 to 100 mg-N / L, preferably 10 to 100 mg-N / L. To control. By denitrifying in this way, the ozone-treated sludge is efficiently decomposed, and the denitrifying bacteria are grown using the BOD component in the ozone-treated sludge as a hydrogen donor, and nitrate ions and nitrite ions are nitrogenated. Reduce to gas and remove. Nitrogen gas is discharged from the exhaust gas passage 14.
[0036]
The liquid in the tank of the first denitrification tank 1 is introduced into the second denitrification tank 2 from the communication path 15, methanol is injected from the BOD injection path 16, and the denitrification treatment is performed by gently stirring with the stirrer 17 under anaerobic condition. Do. In the second denitrification tank 2, the amount of BOD flowing into the second denitrification tank 2 is adjusted to 2.5 to 4 times the NOx-N load flowing into the second denitrification tank 2 by adjusting the amount of methanol injected. Thus, the NOx-N concentration of the liquid sent from the second denitrification tank 2 to the aeration tank 3 is controlled to be 10 mg-N / L or less, preferably 5 mg-N / L or less. By denitrifying in this way, the denitrifying bacteria grow using methanol as a hydrogen donor, and the remaining nitrate ions and nitrite ions are almost completely reduced to nitrogen gas to remove them. The sludge floating in 4 is prevented. Nitrogen gas is discharged from the exhaust gas passage 18.
[0037]
The denitrification liquid in the second denitrification tank 2 is introduced into the aeration tank 3 from the communication path 20 and subjected to aerobic treatment to decompose residual organic substances.
The aerated liquid is introduced into the sedimentation tank 4 from the communication path 21 and separated into solid and liquid, a part of the separated sludge is returned from the return sludge path 11 to the first denitrification tank 1, and the separated liquid is discharged from the treatment water path 22 as treated water. To do. Since denitrification is almost completely performed in the second denitrification tank 2, sludge floating in the settling tank 4 is prevented, and solid-liquid separation is performed well. For this reason, the quality of treated water does not deteriorate.
[0038]
In the pH adjusting tank 5, another part of the separated sludge separated into solid and liquid in the precipitation tank 4 is introduced from the communication path 23 and adjusted to pH 5 or lower.
In the ozone treatment tank 6, sludge whose pH has been adjusted is introduced from the communication path 24, and this is treated with ozone to solubilize and form BOD. By introducing this ozone-treated sludge into the first denitrification tank 1 from the ozone-treated sludge passage 12 and performing denitrification, the ozone-treated sludge is used as a hydrogen donor necessary for nitric acid respiration of denitrifying bacteria. In addition, when excess sludge arises, it discharges out of the system from the sludge discharge channel 25.
[0039]
Ozone-treated sludge obtained by ozone treatment of biological sludge is introduced into the first denitrification tank 1 from the ozone-treated sludge path 12 because the biological sludge is converted to BOD and has good biodegradability. When the NOx-N concentration is controlled to be the above concentration, the ozone-treated sludge is efficiently decomposed in the first denitrification tank 1 and used as a hydrogen donor necessary for nitrate respiration of the denitrifying bacteria. Therefore, the denitrification step can be performed without adding a substrate such as methanol from outside the system. This reduces the processing cost and reduces the amount of excess sludge. Further, the NOx-N concentration of the liquid sent from the second denitrification tank 2 to the aeration tank 3 is controlled so as to be the above concentration, and further denitrification is performed, so that the remaining nitrate ions and nitrite ions are almost completely removed. Thus, sludge floating in the sedimentation tank 4 is prevented, and treated water with high water quality can be obtained stably.
[0040]
【Example】
Next, the present invention will be described with reference to examples.
[0041]
Test example 1
A test for determining the relationship between the residual NOx concentration in the denitrification tank and the sludge reduction performance was performed using a sealed batch experimental tank. That is, the degradation rate was measured using sludge modified to be biodegradable using ozone, and the relationship between the nitric acid concentration and the sludge degradation rate was clarified. The degradation rate of the modified sludge was determined from the rate of decrease of the injected nitric acid concentration.
The results are shown in FIG.
[0042]
As can be seen from the results in FIG. 2, the degradation rate of the modified sludge is very slow when the nitric acid concentration is less than 5 mg-N / L, and the decomposition rate increases with increasing nitric acid concentration in the range of 5 to 10 mg-N / L. It rises and becomes a substantially constant decomposition rate at 10 mg-N / L or more, and even if the nitric acid concentration is increased further, no increase in the reformed sludge decomposition rate is observed.
From this test result, by maintaining the residual NOx concentration in the denitrification tank at 5 mg-N / L or more, preferably 10 mg-N / L or more, the decomposition of the modified sludge proceeds efficiently and sludge reduction is performed well. Can be determined.
[0043]
Example 1 and Comparative Example 1
Using the apparatus shown in FIG. 1, the denitrification treatment of the stainless steel waste water shown in Table 1 was performed. However, the sludge to be treated with ozone was extracted from the second denitrification tank 2 instead of the precipitation tank 4.
The first denitrification tank 1 capacity was 240 L, the second denitrification tank 2 capacity was 330 L, and the aeration tank 3 was 170 L.
[0044]
[Table 1]
Table 1 Wastewater composition

[0045]
The nitrogen-containing wastewater shown in Table 1 was denitrified at a nitrogen load of 670 g-N / d (96 L / h). As a BOD source, methanol corresponding to BOD / N = 2.9 (1940 g-BOD / d) was injected into the first denitrification tank 1 and the second denitrification tank 2 as a whole. The pH of the first denitrification tank 1, the second denitrification tank 2, and the aeration tank 3 was controlled to 7.0 using hydrochloric acid. 800 g-VSS / d of the sludge in the second denitrification tank 2 is continuously drawn out by a pump, adjusted to pH 3 using hydrochloric acid in the pH adjustment tank 5, and then introduced into the ozone treatment tank 6, 1.6 to 2.5 Sludge reforming treatment was performed by reacting with ozone gas of% -ozone / VSS, and the modified sludge was returned to the first denitrification tank 1. The return sludge flow rate from the settling tank 4 was 140 L / h and returned to the first denitrification tank 1.
[0046]
In Example 1, in the first example, methanol was not injected into the first denitrification tank 1, and all 1940 g-BOD / d of methanol was injected into the second denitrification tank 2. At this time, in the first denitrification tank 1 of Example 1, BOD / N = 1.1 as calculated below.
The BOD of the ozone-treated sludge that had been treated with ozone was 0.9 g-BOD / g-VSS as a result of the analysis, so the BOD that was introduced into the first denitrification tank 1 was
0.9 × 800 = 720 g− BOD / d
On the other hand, since the nitrogen load to the first denitrification tank 1 is 670 g-N / d,
BOD / N = 720/670 = 1.1
[0047]
In the second denitrification tank 2, BOD / N = 4.3 as calculated below.
Inflow nitrogen into the second denitrification tank 2 = Measured value of residual nitric acid concentration in the first denitrification tank 1 x (raw water amount + return sludge flow rate)
= 80 mg-N / L x (96 L / h + 140 L / h)
= 18.9 g-N / h
= 450 g-N / d
Methanol injection ratio = 1940 g-BOD / 450 g-N
= 4.3
[0048]
On the other hand, as Comparative Example 1, instead of the first denitrification tank 1 and the second denitrification tank 2 of Example 1, the same treatment as that of Example 1 was performed using an apparatus replaced with one tank of 570 L capacity. . Methanol was injected into the denitrification tank in the same amount (1940 g-BOD / d) as in Example 1. The methanol injection ratio at this time was BOD / N = 2.9.
[0049]
In both Example 1 and Comparative Example 1, the respective operating conditions were kept constant, and continuous operation was performed for 30 days to compare nitrogen removal performance and sludge reduction performance. The seed sludge was collected from an actual apparatus for treating the same waste water, and the treatment was started with an initial concentration of 4000 mg-VSS / L. -Out pull disconnect of excess sludge was not carried out. The processing results are shown in Table 2.
[0051]
[Table 2]

[0052]
The results in Table 2, whereas it Comparative Example 1 both nitrogen removal performance and our Example 1 was sufficient, the sludge concentration is maintained concentration at the start of operation in the embodiment 1, Comparative Example 1 In, a significant increase in sludge concentration occurred, and an increase in the sludge interface of the sedimentation tank occurred. If it is left as it is, it is expected that it will not be possible to separate the sludge and treated water, and it was necessary to extract the sludge and treat the excess sludge.
[Brief description of the drawings]
FIG. 1 is a system diagram of a nitrogen-containing wastewater treatment method according to an embodiment.
2 is a graph showing the results of Test Example 1. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 1st denitrification tank 2 2nd denitrification tank 3 Aeration tank 4 Precipitation tank 5 pH adjustment tank 6 Ozone treatment tank 10 Raw water channel 11 Return sludge channel 12 Ozone

treatment sludge channel

13, 17 Stirrer 14, 18

Exhaust gas channels

15, 20, 21, 23, 24 Connecting path 16 BOD injection path 22 Treatment water path 25 Sludge discharge path

Claims

In a method for treating nitrogen-containing wastewater containing NOx-N in a nitrogen removal system including a denitrification tank,
A denitrification step of biologically denitrifying nitrogen by introducing wastewater containing NOx-N into two or more denitrification tanks;
A part of the biological sludge generated in the nitrogen removal system is pulled out to be easily biodegradable, and then the modified sludge is introduced into the denitrification tank.
Drainage and reformed sludge are introduced into the previous denitrification tank of the denitrification process, and the BOD inflow is controlled to be 2.5 times or less of the NOx-N load flowing into the denitrification tank. Denitrification so that the NOx-N concentration of the nitrogen tank effluent is 5 to 100 mg-N / L,
Introduce the denitrification liquid of the first-stage denitrification tank into the second-stage denitrification tank, and inject a substrate other than the modified sludge as a BOD source, and the NOx-N concentration of the second-stage denitrification tank effluent is 10 mg-N / L or less A method for treating nitrogen-containing wastewater that denitrifies residual NOx-N so that

The treatment method according to claim 1, wherein the inflow amount of BOD into the denitrification tank is adjusted by the inflow amount of the modified sludge and / or the injection amount of the substrate.

The processing method according to claim 1 or 2, wherein the reforming treatment is an ozone treatment.

A denitrification process is performed using at least two denitrification tanks provided in series, and denitrification is performed so that the NOx-N concentration of the denitrification tank effluent is 5 to 100 mg-N / L in the previous denitrification tank. The treatment method according to any one of claims 1 to 3, wherein denitrification is performed so that the NOx-N concentration of the denitrification tank effluent is 10 mg-N / L or less in the subsequent denitrification tank.